Almost all cars need valve replacement after around 50k km if used on CNG. This issue is pretty common on Suzuki cars. If you do not want to go through this expense, drive your car for the first 10,000km on petrol and only then convert to CNG. After converting, drive atleast 100km on petrol for each 1000km on CNG. This way, your valves should last around 80,000km.